BALB/c mice have been immunized with retroviruses and used to produce hybridomas synthesizing monoclonal antibodies reactive to interspecies viral determinants. To date, all of the monoclonal antibodies produced which appear, in solid phase immunoassays, to react with two or more retroviruses, are IgM molecules which fail to immunoprecipitate any antigen. This suggests that antibodies having both interspecies reactivity and the necessary avidity for antigen are rare. A biochemical and immunological characterization of a retrovirus recently isolated from goats has been initiated. This virus, termed caprine arthritisencephalitis virus (CAEV), causes leukoencephalitis, interstitial pneumonia and chronic arthritis. We have shown that CAEV is morphologically and immunologically related to visna, a slow virus of sheep.